Hereinafter, a conventional network system will be described.
FIG. 13 is a block diagram illustrating a loop type network system disclosed in Japanese Published Patent Application No. Hei.9-289518. In FIG. 13, reference numeral 1301 designates a master node, numeral 1302 designates slave nodes, numeral 1303 designates a token-output/analysis-execution unit, numeral 1304 designates analysis execution units, and numerals 1305 and 1306 designate switches which disconnect the network to make connections to the token output unit and the analysis execution unit, whereby the master node 1301 and the slave nodes 1302 are connected in a loop. The switches of the respective nodes are usually placed at A to connect the network.
The master node 1301 places the switch 1305 at B to select the token-output/analysis-execution unit 1303, and issues a token packet which specifies a transmission node and a receiving node for data communication. The token packet which has made a round of the loop is discarded by the mater node 1301, and then the master node 1301 places the switch 1305 to A.
The node which is directed to transmit data by the token packet (transmission node) places the switch 1305 or 1306 at B to output a data packet. When the data packet has made a round of the loop, the node discards the data packet in itself, and returns the switch to A. On the other hand, the node which is directed to receive data by the token packet (receiving node) captures the data packet which is output to the loop-shaped network by the transmission node, into the analysis execution unit 1303 or 1304.
Data communication between the nodes is performed such that the master node successively outputs token packets.
In the above-described structure, however, since the token packet and the data packet are discarded by the nodes which have output these packets, when the number of nodes connected or the total extension of the loop is increased, the transmission time of the packet around the loop increases, resulting in a reduction in the transmission bandwidth. Further, since the node ID of one node should not be matched with the node ID of another node, management of node IDs is difficult when addition of nodes or the like is carried out.